In internal combustion engines, a metal gasket is interposed between respective joint surfaces between a cylinder block and a cylinder head. The metal gasket is fastened in place by head bolts and serves to seal fuel gas, coolant, and lubricating oil in their proper locations in the engine. In particular, sealing of cylinder holes is crucial, because insufficient sealing of the cylinder holes can cause unwanted leakage flows of fuel gas between adjacent cylinders, which can reduce engine power. Leaking fuel gas can also flow into coolant holes circumferentially formed around the cylinder holes, resulting in malfunctions such as engine overheating, or in the worst case, engine seizure.
In respective efforts to prevent the above phenomena, Patent Literatures 1, 2 disclose respective metal gaskets including a resilient metal layer in which annular beads are provided around the cylinder holes, for example. The metal gaskets disclosed in Patent Literatures 1 and 2 include: a metal substrate made of a metal layer and having cylinder holes formed in correspondence with cylinder bores in a cylinder block of an internal combustion engine. Annular beads are formed around the peripheries of the cylinder holes. Coolant holes are formed along peripheries of the annular beads in correspondence with the cooling water jacket of the cylinder block and coolant holes of the cylinder head. An annular step adjustment plate is provided to face the annular beads of the substrate. The step adjustment plate is provided with claw-shaped portions that are inserted into fastening holes formed in the substrate and folded over to one side so that the step adjustment plate is integrated with the substrate. Due to a thickness difference created by the step adjustment plate provided between the annular beads and outer beads, surface pressure as applied to the gasket is appropriately adjusted.
However, one problem with the technologies disclosed in Patent Literatures 1 and 2 is that, as shown in FIG. 18, the claw-shaped portions 3h are each folded over only to one side. Consequently, the step adjustment plate 3 cannot be strongly fastened to the substrate 2. Another problem is that, as shown in FIG. 19, whenever a plurality of step adjustment plates 3 are stacked on top of one another as inventory products or as components required in a process for manufacturing the metal gaskets, wherein the claw-shaped portions 3h should be perpendicularly bent upward from the step adjustment plate 3, some of the claw-shaped portions 3h actually become obliquely bent. These obliquely bent claw-shaped portions 3h can result in increased manufacturing man-hours being required to reshape the step adjustment plates 3 for assembly to the substrate 2. Sometimes, these step adjustment plates 3 are scrapped as being defective when in fact they are not.
In view of the above problems, the present inventors conducted studies revealing that, as shown in FIGS. 20A and 20B, a strong coupling of the step adjustment plate 3 with the substrate is achieved by providing respective prepared holes 3c in protruding pieces 3b projecting from peripheral edges of the step adjustment plate 3. Material drawing is applied to the periphery of the prepared holes 3c using a die including a cylindrical punch so as to form respective tubular portions 3d each having a circular cross section. The tubular portions 3d are inserted through respective fastening holes 2h in the substrate, and the tubular portions 3d are folded over onto the outside surface of the substrate and flattened. As shown in FIG. 21, it has been found that the above structure eliminates the aforementioned need to make alterations to the step adjustment plates at the time of their assembly to the substrate 2. The above structure also solves the problem in which the step adjustment plates 3 are scrapped as being defective (when in fact they are not) because the formed tubular portions 3d have become obliquely bent when multiple step adjustment plates 3 have been stacked on top of one another as manufacturing inventory or otherwise as components required for manufacturing the metal gaskets.